Regulation Mechanism Of Cadmium Species In Biological Soil Crust Of Paddy Field By Fertilization

Consequently,due to the serious pollution of heavy metal cadmium in farmland in China,it poses a threat to food safety and human body.There have been a lot of studies on the heavy metal species of farmland soil in the process of Cd migration and transformation,but few studies have been done on the biological part of paddy crust,which is commonly seen in farmland and attached to the surface of soil.In this paper,three different fertilizers,including KH2PO4,K₂HPO₄ and CO（NH₂）₂,were selected to study the adsorption characteristics of Cd and the transfer and transformation of Cd in the soil crust of paddy fields under the action of these three fertilizers.Among them,the adsorption experiment studied the composition and morphology of heavy metal Cd in the biological soil crust of rice field under different fertilizers and different amounts of heavy metal Cd,and determined and analyzed the p H,conductivity and Cd morphology in the experimental system.In this study,similar methods of soil heavy metal analysis were used to classify and analyze the forms of heavy metals in the biological soil crust of paddy fields.The adsorption and morphology analysis were combined to study the influence of different fertilizers（KH2PO4,K₂HPO₄ and CO（NH₂）₂,）on the adsorption performance of cadmium in the soil crust.Therefore,the research on the stability regulation of heavy metal Cd on the biological soil crust in rice field is further developed.The following conclusions are mainly drawn:Adsorption test results show that:（1）Under the action of KH2PO4,potassium K₂HPO₄ and CO（NH₂）₂,biological soil crust had different effects on Cd adsorption.Among them,KH2PO4 and KH2PO4,mainly inhibited the adsorption of paddy field crust to Cd,while CO（NH₂）₂ promoted the adsorption of paddy field crust to Cd to different degrees.（2）experiments with different concentrations of KH2PO4 showed that KH2PO4 inhibited Cd adsorption by paddy soil crust,and the distribution of Cd in paddy soil crust was mainly oxide binding（61-76%）,followed by residue binding（11-24%）,and finally adsorption（10-14%）.Under the influence of KH2PO4,the content of Cd in paddy soil crust decreased successively,mainly affecting the Cd of manganese oxidation state.After the addition of 10 mmol/L KH2PO4,the content of manganese oxide bound Cd in paddy soil crust decreased significantly,from 5.96 mg/kg at the beginning to 3.80 mg/kg at the end,and the residue Cd increased from 0.89 mg/kg to 1.66 mg/kg.Under the influence of 2-10mmol/L KH2PO4,the change of Cd in the adsorption part of the surface is relatively small,about 0.7-0.9 mg/kg.（3）experiments with K₂HPO₄ at different concentrations showed that K₂HPO₄ inhibited the adsorption of Cd by crust in rice fields,mainly affecting Cd in residue state.Under the influence of K₂HPO₄,the content of Cd in paddy soil crust decreased successively.Among them,after 10mmol/L K₂HPO₄ was added,the content of Cd in residue state binding in paddy soil crust decreased significantly,from the initial 2.56 mg/kg to the final 0.30 mg/kg,and the content of Cd in manganese oxide state decreased from 1.42 to 0.26 mg/kg.Under the influence of 2-10 mmol/L K₂HPO₄,the change of Cd in the biosorption part was relatively large,from 4.6 mg/kg to 2.7 mg/kg.Cd in manganese oxide and residual part of the changes are relatively small.（4）experiments with CO（NH₂）₂ at different concentrations showed that CO（NH₂）₂ had different degrees of adsorption on Cd in paddy field crust.Among them,Cd in the bio-adsorption state showed a decreasing trend with the increase of CO（NH₂）₂ concentration,while Cd in the residue state increased with the increase of CO（NH₂）₂ concentration to different degrees.Among them,the distribution of Cd in paddy field crust is mainly oxide binding（21%-46%）,followed by residue binding（33-64%）,and finally adsorption（14-40%）.Under the influence of 5 mmol/L CO（NH₂）₂,the content of Cd in rice crust had a promoting effect,reaching the maximum of 9.51 mg/kg,in which the content of Cd in the biosorption state reached 3.52 mg/kg,the content of Cd in the manganese oxidation state reached 4.03 mg/kg,and the content of Cd in the residue state reached 2.10 mg/kg.After the addition of 20mmol/L CO（NH₂）₂,Cd in paddy field crust had inhibitory effect,in which the content of Cd bound in the residue state increased significantly to 4.21 mg/kg,the content of Cd in the biosorption state was only 1.21 mg/kg,and the content of Cd in the manganese oxide binding state was 1.53 mg/kg.